The invention relates to outboard drive units and more specifically to drive units which employ underwater exhaust through the lower gearcase.
It is common practice for outboard motors and marine stern drive units to use exhaust systems routing the exhaust down the drive shaft housing, through the lower gearcase, and out an annular passage in the propeller hub. These units are commonly called through-the-hub-exhaust or jet-prop systems. As outboard motor power has risen to 200 horsepower and beyond, non jet-prop propellers such as that described in U.S. Pat. No. 4,080,099, issued to Richard H. Snyder have proven to be the fastest propellers for use on jet-prop units.
At high speed, 70 to 90 miles per hour, certain conditions apparently can cause the exhaust to suddenly ventilate forward along the low pressure side of the gearcase torpedo when a non jet-prop is used. A reaction then occurs which feeds this exhaust into the low pressure side of the propeller blades. A loss of speed, a loss in the bow lifting capacity of the propeller, and a tendency for the boat to go into a slight left-hand turn results, presuming a right-hand propeller is used. This occurrence has been termed a "blowout".